In modern digital communication systems such as digital satellite systems, a goal is to transport digital data bits representing user information such as video, audio and other data types from a source (or a transmitter) to a destination (or a receiver or even multiple receivers) with a pre-defined maximum transportation error rate or bit error rate (BER). To control the transportation errors, typical transmitters usually add redundant bits into an original data bit stream for transmission. The process of adding redundant bits is called forward error control (FEC) encoding. In an encoded bit stream, it is possible for a receiver to recover original information with an eror rate less than the defined error rate by running an FEC decoding algorithm, even if the transmitted bits are contaminated by extraneous signals (e.g., noise and/or interference) and distorted by non-ideal channel characteristics. For some applications, such as voice communication, a maximum BER of 10−2 is acceptable. For other applications, such as file transfer and most internet traffic, a zero BER is required. When a zero BER is required, data is often re-transmitted if a receiver detects any un-correctable errors. For application such as video conferencing and broadcasting, a BER of less than 10−9 is preferred to provide a smooth display of a video signal on video monitors. Such a BER, is often referred to as a Quasi-Error-Free (QEF) condition.
Even if a QEF is achieved using LDPC codewords, such an encoded signal format is not acceptable for transmission, because receivers can't decode received signals without knowing where each encoded portion (such as an LDPC codeword) starts or ends. Time references (or markers) have been necessary throughout the transmission to help identify the positions of such codewords. Similarly, typical communication systems require that receivers' time and frequency be locked to a transmitter's reference, which is referred to as synchronization.
Furthermore, certain overhead information is typically periodically transferred to enable receivers to properly demodulate transmitted signals, decode signals and abstract user messages. For at least these reasons, typical transmitters insert a synchronization pattern and a header periodically into encoded messages, a process called frame formatting. Typically, a receiver first tries to lock onto a synchronization pattern and then decodes the header and message signal. Frame formatting design is critical to overall system performance and can directly impact the cost of establishing and operating a communication system. Frame formatting design often depends on many factors, such as channel characteristics, modulation type and FEC scheme. A well designed frame format may result in high performance receivers that achieve fast frame acquisition, reliable tracking (time and frequency lock) and improved FEC decoding performance (such as meeting the required BER) with minimum overhead at a low cost.